Phosphors and fluorescent compositions for emission of light under low-velocity electron excitation and fluorescent display devices utilizing the same

ABSTRACT

A phosphor for emission of light under low-velocity electron excitation having the formula: 
     
         (Zn.sub.1-X,Cd.sub.x)S:aLi,bM.sup.I,cX                     (I) 
    
     wherein 0≦x≦1, 0&lt;a&lt;1×10 -2  g/g, b≧0, c&gt;0, M I  is at least one element selected from the group consisting of Na, K, Ag, Rb, Cs, Au and Cu, and X is at least one element selected from the group consisting of Al, Cl, Br and I.

This is a continuation of application Ser. No. 06/810,229, filed Dec.18, 1985.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to phosphors which can emit light of highluminance under low-velocity electron excitation, fluorescentcompositions containing such phosphors, and low-velocity electronexcited fluorescent display devices.

2. Description of the Prior Art

Zinc activated zinc oxide phosphors (ZnO:Zn) are well known as phosphorswhich can emir light of high luminance under excitation by low-velocityelectron beams. When excited by low-velocity electron beams, the ZnO:Znphosphors emit green-white light of high luminance, and they arepractically used for fluorescent screens for low-velocity electronexcited fluorescent display devices.

A low-velocity electron excited fluorescent display device (hereinafterreferred to simply as a "fluorescent display device") comprises,basically, an anodic plate having a fluroescent screen on one sidethereof and a cathode disposed face to face with the fluorescent screenin an evacuated tube, whereby the fluorescent screen on the anodic plateis excited by low-velocity electron beams (usually low-velocity electronbeams at an acceleration voltage of at most 100 V) emitted from thecathode to emit light of high luminance. The fluorescent display deviceshaving fluorescent screens composed of the above ZnO:Zn phosphors arewidely used as display elements for table-top calculators, variousmeters and testers, etc.

In recent years, reflecting the expansion of the field of theapplication of the fluroescent display devices, a variety of emittingcolors are desired for the fluorescent display devices. Variousdevelopments have been made on the phosphors which are capable ofemitting light other than green light under low-velocity electronexcitation. As a result, there have been developed some phosphors whichare capable of emitting colored light other than green light underlow-velocity electron excitation. Among them, there are(Zn_(1-x),Cd_(x))S phosphors represented by a specific example of thecomposition of (Zn_(1-x),Cd_(x))S:Ag.

For the preparation of such (Zn_(1-x),Cd_(x))S phosphors, it is known toemploy NaCl, as disclosed by H. W. Leverenz in "An Introduction toLuminescence of Solids" pages 196-199, John Wiley & Sons, Inc., 1950, orin Japanese Unexamined Pat. Publications Nos. 129480/1980 and167381/1982.

Further, in the above-mentioned Japanese Unexamined Pat. Publication No.167381/1982, it is proposed to increase the Ag content in the phosphorin order to improve the luminance of a low-velocity electron excitedfluorescent display device. On the other hand, Japanese Examined Pat.Publication No. 33153/1984 proposes to use a fluorescent compositioncomposed of a mixture of a phosphor and a conductive material such asIn₂ O₃ having a small particle size in order to achieve the same object.

However, it is now desired to further improve the luminance of the lightemitted by a fluorescent display device during its use underlow-velocity electron excitation i.e. at a low voltage and low electricpower, in connection with a recent tendency for color display in thefluorescent display devices and an expansion of the field in whichfluroescent display devices are used.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a phosphor which iscapable of emitting light of high luminance under low-velocity electronexcitation at an acceleration voltage of at most 1 KV, preferably armost 100 V.

Another object of the present invention is to provide a fluorescentcomposition which is capable of emitting light of high luminance underlow-velocity electron excitation.

A further object of the present invention is to provide a fluorescentdisplay device which emits light of high luminance under low-velocityelectron excitation.

The present inventors have conducted extensive researches particularlyon the above-mentioned (Zn_(1-x),Cd_(x))S phosphors among phosphorswhich are capable of emitting light when excited by low-velocityelectron beams (hereinafter referred to as a "phosphor for emission oflight under low-velocity electron excitation") in order to achieve theabove object. As a result, it has been found that when excited bylow-velocity electron beams, (Zn_(1-x),Cd_(x))S phosphors containinglithium (Li) exhibit emission of light with luminance far superior tothe conventional phosphors containing no lithium despite their luminanceunder high velocity electron excitation is substantially equal orinferior to the luminance of the conventional phosphors. The presentinvention is based on this discovery. This discovery is extremelysignificant in that the addition of lithium provides a characteristicwhich is fundamentally different from the characteristics known before.

The phosphor for emission of light under low-velocity electronexcitation of the present invention based on such a discovery, isrepresented by the formula:

    (Zn.sub.1-X,Cd.sub.x)S:aLi,bM.sup.I,cX                     (I)

wherein 0≦x≦1, 0<a<1×10⁻² g/g, b≧0, c>0, M^(I) is at least one elementselected from the group consisting of Na, K, Ag, Rb, Cs, Au and Cu, andX is at least one element selected from the group consisting of Al, Cl,Br and I.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph showing the relation between the lithium content in aphosphor and the luminance and the relation between the lithium contentand the change of the threshold voltage in a fluroescent display devicehaving a fluorescent screen wherein a (Zn₀.65,Cd₀.35)S:Li,Ag,Cl phosphoris used as the phosphor for emission of light under low-velocityelectron excitation of the present invention.

FIG. 2 is a graph showing the relation between the lithium content in aphosphor and the relative luminance under excitation at an excitingvoltage of 30 V in a fluorescent display device having a fluorescentscreen made of a fluorescent composition composed of a mixture of a(Zn₀.70,Cd₀.30)S:Li,Ag,Cl phosphor and In₂ O₃ in a weight ratio of 9:1as the fluroescent composition of the present invention.

FIGS. 3 and 4 are diagrammatic views illustrating fluorescent displaydevices of the present invention. FIG. 3 illustrates a diode, and FIG. 4illustrates a triode.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In the phosphors of the present invention, the desired improvement inthe luminance is obtainable even when the lithium content is very smallat a level almost equal to the limit for the detection by theconventionally available technique for analysis (1 ppm or less). Thetype and the combination of lithium salts to be used may optionally beselected. The upper limit for the lithium content is at a level of 10⁻²g/g (10,000 ppm) in view of the production technique presentlyavailable.

The lithium content is preferably within a range of from 1 to 7100 ppmrelative to the phosphor matrix. More preferably, the content is withina range of from 35 to 5000 ppm.

Lithium may be incorporated not only as a simple substance but also inan optional form and number of lithium compounds such as metal salts,halides (Br,I) or oxides.

The phosphor of the present invention is usually prepared by thefollowing method. Namely, a predetermined amount of an activator or aco-activator such as a monovalent halide (such as AgCl) or a trivalenrmetal salt (such as Al(NO₃)₃) is added to a powder mixture of sulfidesobtained by mixing zinc sulfide (ZnS) powder and cadmium sulfide (CdS)powder in such a ratio that the ZnS powder is (1-x) mol relative to xmol of the CdS powder, and a suitable amount of a lithium source such aslithium chloride (LiCl) is further added, and the mixture is fired in asulfurizing atmosphere such as a hydro-sulfide atmosphere or a sulfuratmosphere ar a temperature of from 500° to 1200° C. for from 0.2 to 5hours, then thoroughly washed with a solvent such as water, dehydratedand dried.

FIG. 1 is a graph showing the relation between the lithium content in aphosphor and the characteristic values in a fluorescent display devicehaving a fluorescent screen wherein a (Zn₀.65 Cd₀.35)S:Li,Ag,Cl phosphoris used as an example of the phosphor for emission of light underlow-velocity electron excitation of the present invention thus obtained.

In this Figure, curves a₁, b₁ and c₁ correspond to the above phosphorprepared by an addition of LiCl as the lithium salt, and curves a₂, b₂and c₂ correspond to the above phosphor prepared by an addition of amixture of NaCl, LiCl, Li₂ SO₄ H₂ O and Li₃ PO₄.1/2H₂ O in an equalmolar ratio as a mixture of lithium salts. Curves a₁ and a₂ indicate thechanges in the threshold voltage (i.e. the applied voltage required toobtain a luminance of 0.1 cd/m²) depending upon the lithium content. Onthe other hand, curves b₁ and b₂ indicate the changes in the luminancedepending upon the lithium content when the applied voltage was 50 V.Likewise, curves c₁ and c₂ indicate the changes of the luminancedepending upon the lithium content when the applied voltage was 12 KV.In this Figure, the value at the lithium content 0 in each curve is thevalue measured by using the conventional phosphor containing no lithium.As shown in the Figure, by the incorporation of lithium into thephosphor, a decrease in the threshold voltage and an improvement in theluminance under low-velocity electron excitation were observed.

The fluorescent composition capable of emitting light under low-velocityelectron excitation according to the present invention is a mixture ofthe above-mentioned phosphor for emission of light under low-velocityelectron excitation of the present invention and a conductive material.The conductive material is at least one member selected from the groupconsisting of metal oxides and metal sulfides, such as In₂ O₃, SnO₂,ZnO, CdS, In₂ S₃, Cu₂ S or Li₂ S. There is no particular restriction asto the amount of the incorporation of the conductive material. However,from the practical point of view, the conductive material isincorporated usually in an amount of from 0.1 to 50% by weight relativeto the phosphor.

For instance, In₂ O₃ to be used for the fluorescent composition of thepresent invention, may be the one which is industrially available.However, it is also possible to employ fired In₂ O₃ obtained by firingan indium compound readily convertible into In₂ O₃ at a high temperaturesuch as a sulfate, a nitrate or a chloride in the air, in an inertatmosphere or in a weakly reducing atmosphere at a temperature of atmost 1500° C., or pulverized In₂ O₃ obtained by adequately pulverizingsuch fired In₂ O₃. The average particle size of In₂ O₃ is preferably atmost 10 μm. Further, a substance which is capable of forming In₂ O₃under heating may preliminary be deposited on the phosphor surface, andthe mixing of In₂ O₃ with the phosphor may be accomplished during thefiring process.

FIG. 2 is a graph showing the relation between the lithium content in aphosphor and the relative luminance under excitation at an excitingvoltage of 30 V in a fluorescent display device having a fluorescentscreen formed by a fluorescent composition composed of a mixture of a(Zn₀.70,Cd₀.30)S:Li,Ag,Cl phosphor and In₂ O₃ in a weight ratio of 9:1.

In this Figure, curve a corresponds to the fluorescent compositionprepared by an addition of LiCl as the lithium salt. Likewise, curve bcorresponds to the fluorescent composition prepared by an addition of amixture of NaCl, LiCl, Li₂ SO₄.H₂ O and Li₂ PO₄.1/2H₂ O in an equalmolar ratio as the mixture of lithium salts. The sample for the relativeluminance of 100% is the fluorescent composition containing theconventional phosphor with the lithium content 0. As shown in thisFigure, by the incorporation of lithium to the phosphor, an improvementin the relative luminance was observed.

The fluorescent display device of the present invention is characterizedin that it contains the phosphor of the present invention in itsfluorescent screen. Of course, it may contain the above-mentionedfluorescent composition of the present invention in its fluorescentscreen.

FIGS. 3 and 4 are diagrammatic views illustrating typical examples ofthe fluorescent display devices of the present invention. FIG. 3illustrates a diode, and FIG. 4 illustrates a triode. As shown in FIGS.3 and 4, in these fluorescent display devices, a fluorescent screen 12is formed on one side of an anodic plate 11 made of e.g. an aluminumplate. The anodic plate 11 is supported by a ceramic substrate 13. Acathode 14 is provided face to face with the above fluorescent screen 12formed on one side of the anodic plate 11 The fluorescent screen 12emits light when excited by low-velocity electron beams emitted fromthis cathode 14. Particularly, in the case of the triode of FIG. 4, agrid electrode 15 is provided in the space between the cathode 14 andthe fluorescent screen 12 to control or diverge the low-velocityelectron beams emitted from the cathode 14. In the fluorescent displaydevices shown in FIGS. 3 and 4, only one cathode 14 is used. However, inthe case where the fluorescent screen 12 has a wide surface area, two ormore cathodes may be provided. There is no particular restriction as tothe number of cathodes. The anodic plate 11 having a fluorescent screen2 on one side thereof, the ceramic substrate 13 and the cathode 14 (FIG.3), or the anodic plate 11 having a fluorescent screen 12 on one sidethereof, the ceramic substrate 13, the cathode 14 and the grid electrode15 (FIG. 4), are all sealed in a transparent container 16 made of e.g.glass, and the interior 17 is maintained to be in a high vacuumedcondition of at least 10⁻⁷ Torr. The fluorescent screen on the anodicplate is a flat plate, whereas the cathode is a wire-like. It istherefore preferred to provide a mesh grid electrode as a divergingelectrode between the cathode and the fluorescent screen, as shown inFIG. 4. In this case, good results can be obtained when the meshopenings are made as small as possible so that the loss of luminance ofthe fluorescent screen is minimized and the low-velocity electron beamscan efficiently be diverged. Specifically, the mesh diameter ispreferably at most 500 μm, and the opening rate (the surface area of theopenings for passage of the low-velocity electron beams relative to thetotal surface area of the grid electrode) is preferably at least 50%.

The anodic plate is divided in its electrode mode into sections requiredfor the representation of characters or designs so that a voltage canselectively be applied to the respective electrode sections to displayan optional character or design. Further, it is possible to obtain afluorescent display device capable of displaying a multi-coloredrepresentation, if the anodic plate is divided into dots or into lines,a phosphor screen containing the phosphor for emission of light underlow-velocity electron excitation of the present invention is formed on apart of such divided electrodes and a phosphor screen containing aphosphor for emission of light under low-velocity electron excitationhaving an emitting color different from the emitting color of the abovephosphor is formed on the other part of electrodes.

The fluorescent display devices of the present invention may beprepared, for instance, by the following method.

Firstly, the above-mentioned phosphor or fluorescent composition of thepresent invention is mixed with a suitable organic binder to prepare anink containing the phosphor. Then, the ink is poured onto a silk screenplaced on the anode, and a fluorescent screen is formed on the anodicplate by rubbing the silk screen with a squeegee. The fluorescent screenthus formed is baked in the air to decompose the organic binder presentin the fluorescent screen. The process for the preparation of thefluorescent screen for the fluorescent display devices of the presentinvention is not restricted to such a screen printing method. Further, aplurality of phosphors (including phosphors other than those of thepresent invention) may be incorporated into the fluorescent screen. Thematerial of the anodic plate may not necessarily be uniform, and may,for example, be formed with a variety of patterns. However, the anodicplate is preferably flat. Then, a cathode made of e.g. a tungsten wireheater coated with an electron emitter such as BaCO₃ or SrCO₃, isdisposed face to face with the fluorescent screen on the anodic platewith a space of not more than 5 mm. Then, the pair of such electrodesand a getter such as Ba or Ti are placed in a transparent container madeof e.g. glass, and the gas in the container is subjected to baking, andwhile vacuuming the container by a vacuum pump such as a rotary pump,electricity is applied to the cathode to activate the electron emitter.When the interior of the container reached to a vacuumed level of atleast 10⁻³ Torr, the sealing of the container is conducted. After thesealing, the getter is emitted to further increase the degree of vacuumin the container. Thus, a fluorescent display device of the presentinvention is obtained.

As mentioned in the foregoing, the present invention provides aphosphor, a fluorescent composition and a fluorescent display devicewhich is capable of providing remarkably high luminance underlow-velocity electron excitation at an acceleration voltage of at most 1KV, particularly at most 100 V, by incorporating lithium into the(Zn_(1-x),Cd_(x))S phosphors. Thus, the industrial value of the presentinvention is extremely high.

Now, the present invention will be described with reference to theExamples. However, it should be understood that the present invention isby no means restricted to these specific Examples.

EXAMPLE 1

The following materials were thoroughly mixed.

    ______________________________________                                        Zinc sulfide (ZnS), phosphor grade                                                                      137.5  g                                            reagent                                                                       Cadmium sulfide (CdS), phosphor grade                                                                   87.5   g                                            reagent                                                                       CdS containing 2000 ppm of silver                                                                       25     g                                            Lithium chloride (LiCl), special grade                                                                  2.5    g                                            reagent                                                                       ______________________________________                                    

The mixture was placed in a quartz crucible, then fired in an electricfurnace at 800° C. for 2 hours, subjected to levigation, washed withwater until the electric conductivity of the supernatant became 3 μs/cm,dehydrated and dried to obtain a (Zn₀.65,Cd₀.35)S:Li,Ag,Cl phosphorcontaining 6.5 ppm of lithium relative to 1 g of (Zn₀.65,Cd₀.35)S:Ag,Cl.

This phosphor was mixed with a binder comprising ethyl cellulose andcarbitol to obtain an ink, which was then coated on a plate by means ofa 250 mesh silk screen, then heated at 450° C. for 30 minutes, and driedat 100° C. for 30 minutes to form a fluorescent screen A.

On the other hand, a Zn₀.65,Cd₀.35)S:Ag,Cl phosphor containing nolithium was separately prepared, and a fluorescent screen A' was formedin the same manner as the preparation of the above fluorescant screen.

The fluorescent screens A and A' were placed in a vacuumed container,and excited by low-velocity electron beams at 50 V, whereby thefluorescent screen A showed a relative luminance of a level about 30times the luminance of the fluorescent screen A'.

EXAMPLE 2

A ZnS:Li,Cl phosphor containing 320 ppm of lithium (added in the form ofLiCl) relative to 1 g of ZnS:Cl, was prepared. On the other hand, aZnS:Cl phosphor containing 10 ppm of lithium relative to 1 g of ZnS:Clwas prepared for the purpose of comparison. By using these phosphors,fluorescent screens were prepared, then placed in a vacuumed containerand excited by low-velocity electron beams at 50 V in the same manner asin Example 1, whereby the ZnS:Li,Cl fluorescent screen exhibited aluminance of a level 5 times the luminance of the ZnS:Cl fluorescentscreen.

EXAMPLE 3

A (Zn₀.9,Cd₀ 1)S:Li,Au,Al phosphor containing 1000 ppm of lithium (LiCl,Li₂ SO₄.H₂ O and Li₃ PO₄.1/2H₂ O are added in equimolar amounts)relative to 1 g of (Zn₀.9,Cd₀.1)S:Au,Al, was prepared. On the otherhand, a (Zn₀.9,Cd₀.1)S:Au,Al phosphor containing no lithium was preparedfor the purpose of comparison. By using these phosphors, fluorescentscreens were prepared and excited by low-velocity electron beams at 50 Vin a vacuumed container in the same manner as in Example 1, whereby the(Zn₀.9,Cd₀.1)S:Li,Au,Al fluorescent screen exhibited a luminance of alevel 3.5 times the luminance of the (Zn₀.9,Cd₀.1)S:Au,Al fluorescentscreen.

EXAMPLE 4

A (Zn₀.7,Cd₀.3)S:Li,Na,Ag,Al phosphor containing 400 ppm of lithium(LiCl, Li₂ SO₄.H₂ O and Li₃ PO₄.1/2H₂ O were added in equimolar amounts)relative to 1 g of (Zn₀.7,Cd₀.3)S:Ag,Al and 7 ppm of Na, was prepared.On the other hand, a (Zn₀.7,Cd₀.3)S:Ag,Al phosphor containing no lithiumwas prepared for the purpose of comparison. By using these phosphors,fluroescent screens were prepared and excited by low-velocity electronbeams at 50 V in a vacuumed container in the same manner as in Example1, whereby the (Zn₀.7,Cd₀.3)S Li,Ag,Al fluorescent screen exhibited aluminance of a level 4 times the luminance of the (Zn₀.7,Cd₀.3)S:Ag,Alfluorescent screen.

EXAMPLE 5

A fluorescent composition prepared by mixing 9 parts by weight of thelithium-containing phosphor of Example 1 and 1 part by weight of In₂ O₃,was screen-printed to form a fluorescent screen B. On the other hand, afluorescent screen B' was prepared in the same manner as above by usinga fluorescent composition prepared by mixing 9 parts by weight of thephosphor of Example 1 containing no lithium and 1 part by weight of In₂O₃ for the purpose of comparison. These fluorescent screens were placedin a vacuumed container, and excited by low-velocity electron beams at30 V, whereby the fluorescent screen B exhibited a luminance of a level9 times the luminance of the fluorescent screen B'.

EXAMPLE 6

A fluorescent composition prepared by mixing 6 parts by weight of thelithium-containing phosphor of Example 2 and 4 parts by weight of In₂O₃, was screen-printed to form a fluorescent screen C. On the otherhand, a fluorescent screen C' was formed in the same manner as above byusing a fluorescent composition prepared by mixing 6 parts by weight ofthe phosphor of Example 2 containing no lithium and 4 parts by weight ofIn₂ O₃ for the purpose of comparison. These fluorescent screens wereplaced in a vacuumed container, and excited by low-velocity electronbeams at 30 V, whereby the fluorescent screen C exhibited a luminance ofa level 5 times the luminance of the fluorescent screen C'.

EXAMPLE 7

A fluorescent composition prepared by mixing 8 parts by weight of thelithium-containing phosphor of Example 3 and 2 parts by weight of In₂O₃, was screen-printed to form a fluorescent screen D. On the otherhand, a fluorescent screen D' was prepared in the same manner as aboveby using a fluorescent composition prepared by mixing 8 parts by weightof the phosphor of Example 3 containing no lithium and 2 parts by weightof In₂ O₃ for the purpose of comparison. These fluorescent screens wereplaced in a vacuumed container, and excited by low-velocity electronbeams at 30 V, whereby the fluorescent screen D exhibited a luminance ofa level 10 times the luminance of the fluorescent screen D'.

EXAMPLE 8

A fluorescent composition prepared by mixing 99 parts by weight of thelithium-containing phosphor of Example 4 and 1 part by weight of In₂ O₃,was screen-printed to form a fluorescent screen E. On the other hand, afluorescent screen E' was prepared in the same manner as above by usinga fluorescent composition prepared by mixing 99 parts by weight of thephosphor of Example 4 containing no lithium and 1 part by weight of In₂O₃ for the purpose of comparison. These fluorescent screens were placedin a vacuumed container, and excited by low-velocity electron beams at30 V, whereby the fluorescent screen E exhibited a luminance of a level10 times the luminance of the fluorescent screen E'.

EXAMPLE 9

Fluorescent display devices were prepared by using the fluorescentscreens of Examples 5 to 8 as the anode and a filament coated with aelectron beam emitting substance as the cathode, and their luminance wasmeasured respectively, whereby the same results as in Examples 5 to 8were obtained.

What is claimed is:
 1. A low-velocity electron excited fluorescentdisplay device, comprising a sealed evacuated tube, an anodic plateenclosed in said tube having a fluorescent screen on one side thereof,and a cathode enclosed in said tube standing face-to-face with saidfluorescent screen, wherein said fluorescent screen contains a phosphorwhich is capable of emitting light of high finance under low-velocityelectron excitation having the formula:

    (Zn.sub.1-x Cd.sub.x)S:aLi,bMI,cX,dY

wherein 0≦x≦1, a is such that Li is present in the amount of about1-10,000 ppm, b≧0, c≧0, and d≧0, with the proviso that c+d>0, andwherein M^(I) is at least one element selected from the group consistingof Na, K Ag, Rb, Cs, Au and Cu; X is at least one element selected fromthe group consisting of Cl, Br and I; and Y is Al.
 2. The low-velocityelectron excited fluorescent display device of claim 1, wherein saidphosphor contains Li in the amount of about 35-5,000 ppm.
 3. Thelow-velocity electron excited fluorescent display device of claim 1wherein said element M^(I) is Ag, Au or Na and Ag.
 4. The low-velocityelectron excited fluorescent display device of claim 1, wherein saidelement X is Cl.
 5. The low-velocity electron excited fluorescentdisplay device of claim 1, wherein said fluorescent screen comprises afluorescent composition comprising a phosphor which is capable ofemitting light of high luminance under low-velocity electron excitationhaving the formula:

    (Zn.sub.1-x, Cd.sub.x)S:aLi, b M.sup.I, cX, dY

wherein 0≦x≦1, a is such that Li is present in the amount of 1-10,000ppm, b≧0, c≧0 and d≧0, with the proviso that c+d is greater than 0, andwherein M^(I) is at least one element selected from the group consistingof Na, K, Ag, Rb, Cs, Au and Cu, X is at least one element selected fromthe group consisting of Cl, Br and I; and Y is Al, and at least oneconductive material selected from the group consisting of conductivemetal oxides and conductive metal sulfides.
 6. The low-velocity electronexcited fluorescent display device of claim 5, wherein said conductivematerial is present in the amount of 0.1-50% by weight relative to saidphosphor.